1. Importance of Fruits in Human Diet
As “accessory foods,” fruits have an important role in the human diet. They add a variety of color, taste, and texture to meals and snacks. Nutritionally, fruits provide significant amounts of several vitamins and are major contributors of essential minerals.
Most fresh fruits contain 75 to 95 percent water. They are low in protein but generally contain substantial quantities of carbohydrates, including varying proportions of dextrose, fructose, sucrose, and starch, depending on the type of fruit and its maturity.
Dehydration is probably the oldest method of preservation of fruits. Sun-dried fruits antedate the use of fire for cooking and the roots of the modern dehydrated, dehydrofrozen, and granulated fruit industry can be traced back to this oldest method of fruit processing.
Fermentation was another popular mode of fruit juice processing and preservation. Fermented beverages such as beer, wine, cider, brandy, cordials, and nectars have since given way to fruit juices, punches, nectars, sport drinks, and concentrates. These products could be canned, frozen, dried, powdered, or concentrated.
Chemical preservation of whole, sliced, or sectioned fruit by soaking them in honey or sugar syrup or by use of vinegar or wine represents one of the oldest methods of fruit processing and preservation. Today, these products have been replaced by a variety of fruit preserves, jams, marmalades, glazes, jellies, fruit butters, sauces, and pickles.
Thus, the leading methods of processing fruits have shifted continuously through the centuries from sun-drying and sugar and chemical preservation towards modern day, technology-oriented processing methods such as canning, artificial dehydration, concentration, radiation, and freezing, or combinations of one or more of these methods. With that shift, the consumption pattern of fruits has also changed. The percentage of fruits processed as compared to those eaten fresh has steadily increased. This general trend appears to have been due to improvements in varieties of fruits for processing, horticultural practices and production of fruits especially suitable for processing, mechanical harvesting of fruits, techniques for processing large volumes of fruits, containers more suitable for processed fruit, extending the shelf-life of processed fruits, raising the standards and nutritional value of processed fruits, and more attention to marketing of processed fruits, especially for export purposes.
2. Fruit Juice Concentration
The common liquid foods encountered in everyday life are mostly aqueous solutions and/or suspensions. Those prominent in our daily life include fruit juices, soups, tea, coffee, milk, beer, and wines. The solid content of most liquid foods is low, usually in the range of 8 to 16 percent. Oftentimes it is expensive to package, store, and ship “single-strength” liquids, and in many cases it is desirable to remove a part or all of the water from such liquids.
It is obvious that much of what the consumer eats and pays for in food is water. Water alone makes up 70 to 95 percent of the total cost of most products. Another aspect of water in frozen foods can be judged by the energy required for transportation.
Fruit juices are watery mixtures of mostly unstable volatile organic compounds. They are heat sensitive and their color and flavor deteriorate rapidly as processing temperatures are increased. Even at moderate temperatures, many of their components are unstable. At temperatures between 40 and 70° C., enzyme-catalyzed reactions can alter juice properties within a few minutes. In order to inactivate the enzymes, juices must be heat treated. At the same time, to obtain a quality product, it is essential to have sanitary conditions of high standards. Since it is the aroma volatile that give food product their wide variety of flavor sensations, even monic changes in aroma during concentration can greatly alter the sensory qualities of the final product.
Selective water removal processes commonly used in the food industry can be divided broadly into two classes: concentration and dehydration processes. The former includes processes that increase solids content to about 50 to 60 percent (still in liquid form) and the latter those that reduce water content to less than 10 percent and the final product is in solid form. The selection of any one of these processes for removing water from a food product is primarily governed by the physical properties of the form (liquid or solid), economics, and the desired quality of the final products.
A.) Concentration Processes
Concentration processes for fruit juices may be broadly classified based on whether removal of water involves a phase change. Processes which require a phase change include evaporation and distillation, pervaporation, and crystallization or freezing. In direct and reverse osmosis processes, phase change of solvent is not required. Maximum separation of water in evaporation and freezing processes is obtained at phase equilibrium. The rest are typical nonequilibrium processes and are based upon differences in velocity of equilibrium approach.
3. Evaporation and Distillation
Evaporation is defined as the removal by evaporation of a part of the solvent (usually water in case of food liquids) from a solution or dispersion of essentially nonvolatile solutes. The term “evaporation” is usually used when the resultant product is still in a liquid or semisolid state.
Evaporation is probably the oldest method of concentration known to mankind. Some of the most primitive methods of evaporation are still in use today. For example, the use of solar ponds is still an economical means of salt production. At present, evaporation is considered as the best developed, economically the most favorable, and the most widely used method for concentration of food liquids.
For food liquids whose quality is not determined by their aroma composition, evaporation may be conducted at the boiling point of the liquid. The quality of most food liquids, however, is primarily influenced by their aroma characteristics. Almost all aroma and flavor constituents of foods are low-boiling volatile compounds. Thus, they may be removed prior to effecting water removal or may be destroyed depending on their thermal stability. In evaporation processes where the liquid is to be concentrated fourfold or more, the loss of these compounds is almost directly proportional to the loss of vapor. Lower processing temperatures are, therefore, required for such food liquids. The boiling point of these liquids can be lowered by reducing the pressure. Such water removal at reduced pressure, and hence at reduced temperature, is known as vacuum evaporation. Concentration of fruit juices by vacuum evaporation is, however, still a severe process. It is reported that when apple juice was concentrated by heating under the vacuum, the first 10 percent of the juice vaporized contained all the volatile flavoring constituents. Commercially, fruit juice concentrates are, therefore, produced by first stripping their aroma volatile in a distillation column, followed by concentration in a vacuum evaporator. The aroma concentrate is then added back to the concentrate of nonvolatile liquid to yield a flavorful product. A much simpler method to restore partly the quality of the final product is to add fresh juice to the concentrate. The resulting dilution with “cutback” of fresh juice to the concentrate makes it impractical to obtain products above fourfold the original strength.
Fruit juices contain both soluble (such as sugars and acids) and insoluble (such as fiber) components and, therefore, cannot be defined as “true solutions.” In such a system, the thermodynamic properties of water deviate from those of pure water; therefore, the phase diagrams for pure water cannot be applied directly for the purpose of calculating energy requirements for the removal of water. To effect water removal, the total amount of energy input necessary is governed by the following factors: 1) the initial temperature of the feed; 2) operating pressure in a particular system; 3) the amount of water to be removed; 4) the effect of the solubles in solubles present in the feed on the thermodynamic properties of water; and 5) efficiency of a particular process design.
Computing such energy requirements requires knowledge of process design and the equipment used to achieve the desired degree of concentration. In simplistic terms, the minimum energy required to transform unit mass of saturated liquid water to saturated vapor (at a constant temperature and pressure) is equal to the enthalpy or the latent heat of vaporization.
Evaporation may be conducted in a batch or continues mode depending on the need. In the food industry, the latter type is more commonly used. Most vacuum evaporators consist of multistage units. The heating medium for each stage may be stem, water vapor from the previous stage, or both. When water vapor from the previous stage (boiling juice) is used as heating medium to evaporate water from juice at a lower temperature, the process is known as an “effect.” As many as seven stages and four effects have been used to concentrate orange juice. Several aspects of the types of evaporators used in the food industry have been reviewed.
A.) Aroma Recovery in Evaporation Processes
The highly volatile constituents of fruits that are responsible for the characteristic aroma of the fruit juices are present in only trace amounts. Since these aroma compounds are, in general, more volatile than water, they may be lost either partly or completely if the vapor is discarded during concentration of fruit juices by evaporation. The “cutback” process is used to restore partly the quality of concentrated juice that was not suited for the production of concentrates other than citrus juices. With newer, improved technology, the volatile aromas can be recovered by removing them from either the feed before evaporation or from the vapor produced in the evaporator. These dilute aqueous aromas can then be concentrated by distillation or freeze concentration and are returned to the concentrate of nonvolatile from the evaporator. Several reviews have described the aroma recovery systems available for the production of flavorful fruit juice concentrates.